The present invention is directed towards spatial dithering for optical projection displays.
Optical projection systems in which the image is generated by light modulated by one or more xe2x80x9clightvalvesxe2x80x9d are becoming increasingly common. Devices such as televisions, presentation projectors and computer monitors have utilized such lightvalve based projection systems. Typically, in a single lightvalve system, a color image is produced by projecting red (R), green (G) and blue (B) (collectively referred to as xe2x80x9cprimaryxe2x80x9d hereinafter) image fields in a time sequential manner with sufficient rapidity that flicker is not apparent. The overall frame rate desired for color images is typically 60 Hertz or greater. Thus, the corresponding interval between each color image field is {fraction (1/180)}th of a second or less.
Single lightvalve systems are relatively inexpensive and the resulting performance is satisfactory. However, an inherent drawback of time sequential lightvalve and other systems is an effect known as xe2x80x98color breakup artifactxe2x80x99 or xe2x80x98field sequential color artifactxe2x80x99. Color breakup artifact manifests itself to a viewer as a transient rainbow-like fringing effect when rapid eye movements of several degrees are made. The effect is an inherent property of the human visual system but sensitivity to the effect varies greatly from person to person. Moreover, the seriousness of the effect depends strongly on the nature of the image being viewed.
One theory is that increasing the frame rate from 60 Hertz to several hundreds or thousands of Hertz would eliminate color breakup artifact. However, since driving displays at such high frequencies presents complicated and expensive engineering problems, experimental evidence for the increased frame rate theory is difficult to obtain.
An alternative approach is to abandon time sequential imaging while still using only one lightvalve by presenting the primary colors to the viewer in the space domain, rather than in the time domain. One way of constructing such a space sequential system would be to arrange the R, G and B pixels in a mosaic pattern, like the arrangement of phosphor spots in a Cathode Ray Tube device. The lightvalve would be illuminated using white light, and each R pixel would be covered with a red filter, each G pixel with a green filter and each B pixel with a blue filter. The requisite filter array would contain about 106 or more filters. Furthermore, in the case of a micro-display lightvalve array, each filter would measure only 10xc3x9710 xcexcm2. Though conceptually easy, implementing such large filter arrays and such small individual filters could be prohibitively expensive. Disadvantageously, compared to field sequential imaging, mosaic filter arrays need about three times as many pixels.
An approach which does not suffer from the disadvantage of an increased pixel count has also been proposed. Though using a mosaic filter, the filter is xe2x80x98spatially ditheredxe2x80x99 over the lightvalve by approximately plus or minus one pixel horizontally and vertically. For a 60 Hertz frame rate, the xe2x80x98dither ratexe2x80x99 would be about 180 Hertz. A conventional way to achieve dithering is in the use of a spatial multiplexer. A possible disadvantage of the spatial multiplexer is the cost of the device itself. Another is its thickness of 3 to 5 millimeters, which would increase the back focal length required of the projection lens, thereby increasing the cost of the projection optics. A third is that it entails the use of a mosaic filter.
Thus there is a need to implement spatial dithering with a lower cost and complexity.
The invention in one or more embodiments consists of a lenslet array placed immediately in front of a lightvalve, and a segmented active color filter placed at the projection lens"" aperture stop or at any position optically conjugate thereto. Using a segmented color filter that is xe2x80x98activexe2x80x99 (in the sense that the colors of the filter segments are sequenced), the spatial dithering process can be placed under electronic control. In various embodiments, the active color filter can produce any pattern and configuration of colors, which are then repeated throughout the lightvalve. Further, in some embodiments, the lenslet array and lightvalve may be separated from one another.